This invention relates to an apparatus and method for effecting liquid transport in the liquid flow systems commonly used in analytical instruments. More particularly the invention relates to liquid flow systems requiring an essentially constant flow rate therein. Additionally, the invention relates to a coordinated cam-driven scheme to effect the timing and operation of multiple system elements while providing the means for effecting an essentially constant rate of liquid flow within a liquid conduit.
Early liquid handling devices used in analyzers such as the Coulter Counter.RTM. particle analyzers, used a simple vacuum pump, connected to a vacuum bottle to smooth pressure variations, and a series of manually operated stop cock controls to effect the movement of fluids in the instrument. This system is cheap, reliable and of limited use in modern automated instruments.
In the more simple of the analytical devices, a plurality of cams can be positioned on a single shaft and operate upon a single function of the device. In U.S. Pat. No. 4,631,483 to Oscar Proni et al. the power supply is provided in the form of a weight attached to a bellows. The operator lifts the weight and then releases it so that the free fall of the weight expands the bellows and thereby creates a vacuum in the liquid transport system. The purpose for this approach lays in the fact that Proni's system requires a constant or nearly constant pressure on the bellows to produce the liquid flow. The present invention is directed to a system which requires a constant flow rate. Such an analyzer is described in U.S. Pat. No. 5,094,818 to Longman, et al. and is hereby incorporated by reference.
In current automated counters a large capacity pump or pumps are used to generate both pressure and vacuum. The liquid flow is controlled with pinch valves. The timing of the pinch valve operation is accomplished electronically or by a series of cams which operate to turn on and off electrical contacts which in turn operate motors or drivers. The electric drivers can be used to effect operation of the pinch valves. Where multiple mechanical or system functions are integrated within a single device, or where the mechanical or system functions are difficult to synchronize, separate drive systems, operating at separate speeds upon separate mechanical or system functions, is generally required. The net effect of such perceived difficulties has been to design the various subassemblies of an instrument with separate power supplies or drive systems and the integration or synchronization of their various functions with a microprocessor and elaborate software programs. This leads to increased costs, complexity and generation of internal heat.
As mentioned earlier cam activated piston systems are known. However, in systems requiring a constant flow cams and cam followers are not used to effect such a result. For example in an internal combustion engine cams and cam followers are used to time when the valves open and close. The cam scheme in the engine does not operate to control the rate at which these events occur. Other than the obvious rate of once every cycle the cams do not effect the rate at which the valves open and close. The fuel supply system of the internal combustion engine, which requires and controls a constant flow rate within it, is not controlled or effected by the cam system. The fuel system uses an entirely separate system. The same is true in the prior art analyzers. Pistons, cams and cam followers are used for timing but not for creating or controlling a constant flow rate.
U.S. Pat. No. 4,631,483 discussed above recognized that a cam system could be used to replace the function of an operator, that of raising the weight, but to enable the weight to operate properly the cam could not be followed by a cam follower on the release of the weight or down stroke.
It is an object of the present invention to provide a reliable, inexpensively manufactured fluid transport system having an essentially constant flow rate therein.
It is a further object of this invention to eliminate the need for multiple electronic drivers or electronic timing controls in a constant flow rate liquid flow system.